ECLair has 4 general-purpose registers, named A through D. In addition, there are several other registers used to control the CPU itself, and some accessible only from within microcode to handle internal system functions.

General Purpose Registers

• 4 general purpose registers: A, B, C, and D
  • Also accessible as 8-bit halves AL, AH, BL, BH, CL, CH, DL, and DH
• ECLair is load-store architecture, so most operations operate on these registers

Special User-Accessible Registers

• Program Counter: PC
  • Keeps track of what address in memory is currently being executed.
  • Implemented as a counter, microcode can load the counter from Z or increment the counter
  • From userspace, can modify this using jmp instructions
• Stack Pointer: SP
  • Keeps track of where the end of the stack currently is.
  • Microcode can do all normal register operations with this, and it’s used for push/pop operations.
  • From userspace, can load/store this like any other register.
• Page Table Block – PTB
  • Keeps track of which block of the page table we’re currently using. This may get implement as one block per PID once a *NIX port happens, or each process may get more than one block, not sure yet.
  • Microcode and userspace can load this but not read it back
  • Used internally in the paging subsystem to control the page table
• System Flags and Status – STATUS/FLAGS
  • High byte holds various system flags:
    • Bit 9 – Paging Enabled – PE
    • Bit 8 – Mode – M (Supervisor/User)
    • Bit 7 – Interrupts Enabled – IE
  • Low byte holds various system status bits:
    • Bit 2 – Equal – E
    • Bit 1 – Carry/Overflow – CO
    • Bit 0 – Zero – Z

Internal Microcode Registers

• IR – Instruction Register
  • Instruction currently being executed is held in this register
  • Microcode loads this as part of the instruction fetch
• RR – Register Register (a.k.a. IR2)
  • Second byte of the instruction currently being executed is held in this register, if the instruction is a two-byte one
    • Two-byte instructions are ones that affect two different registers
  • Lower nibble is a register specification
  • Upper nibble is currently unused
  • Microcode can use data from this register to specify which register to read
• MAR – Memory Address Register
  • Memory location being addressed is held in this register
  • Can be loaded from Z or PC, drives the address bus (via the paging subsystem if PE flag is on) during memory read/write operations
• MDR – Memory Data Register
  • Contents retrieved from memory or contents about to be written to memory are held in this register
  • Can be loaded from Z (for memory writes) or data bus (for memory reads)
  • When loading from data bus, can load either the high or low byte selectively as the data bus is 8 bits and this register is 16
• RPT – Repeat Register
  • Allows microcode sequences to be executed multiple times
  • Implemented as a counter, can be loaded from Z or decremented, and microcode can branch based on whether RPT=0
• ALU Input Registers: X, Y
  • 16-bits wide each
  • Microcode loads these as needed from the XY bus
  • ALU is connected to these two registers